Applicants have recognized that cylinder-cylinder air/fuel ratio imbalances may occur due to cylinder-to-cylinder variation in intake valve depositions, plugged EGR orifices, and/or shifted fuel injectors.
While various approaches have been set forth for individual cylinder air-fuel control, with the aim at reducing cylinder-cylinder air-fuel ratio variation, such variation may persist. As such, air/fuel imbalance monitoring systems and methods for monitoring air/fuel ratio imbalance of an internal combustion engine having a plurality of engine cylinders are provided herein. An example of the method may include routing exhaust gas from a first group of cylinders to an exhaust gas oxygen sensor, and during selected operating conditions, indicating that air/fuel of at least one cylinder in the first group is imbalanced based on a response of the exhaust gas oxygen sensor at frequencies at or above firing frequency of the cylinders in the first group. An example of the system may include an exhaust gas oxygen sensor positioned in such a way that exhaust gas from the group of engine cylinders are routed to the exhaust gas oxygen sensor, and a controller configured to during selected operating conditions, indicate that air/fuel of at least one cylinder in the group is imbalanced based on a response of the exhaust gas oxygen sensor at frequencies at or above firing frequency of the cylinders in the group. In one particular example, the group of cylinders may be a sub-set of the cylinders in the engine, where the exhaust gas oxygen sensor receives exhaust gas only from the cylinder sub-set.
By basing the indication of air-fuel ratio imbalance on the exhaust gas oxygen sensor response at or above the firing frequency of the cylinders to which the sensor is exposed, it is possible to isolate feedback control interactions with the monitoring function, and thereby achieve a reliable indication of imbalance. Such is the case even in the example where the sensor reading is confounded with exhaust gas from a plurality of cylinders.